A graph or diagram of stress and strain is shown as above. OE Portion is straight line which indicates that stress is proportional to strain. Therefore the wire obeys Hooke’s law up to the point E this point is called elastic limit. EE’ Portion is curved towards strain axis this shows that increase in strain is more, than increase in stress. In this region stress is not proportional to strain. Between any point E and E’ if all load is removed then some permanent elongation / Expansion / increase in length takes place in the wire this is called set. When wire is again loaded, a new straight line SE’ is obtained which obey Hooke’s law. Some portion after the point Y is almost parallel to strain axis this shows that strain increases without increase in stress just like wire flows. This is called plastic flow. The point at which the plastic flow begins is called yield point. During plastic flow the cross-section of wire decreases up to point D. this point D represents the maximum stress which the wire can bear. Finally wire breaks. B is called breaking point.